The invention is based on a sensor arrangement for detecting a wetting of a window, according to the preamble to the main claim.
DE 197 01 258 A1 has disclosed a sensor arrangement for controlling wiper/washer systems for motor vehicle windows, which function in accordance with an optoelectronic principle. The sensor arrangement has a number of transmitters and at least one receiver which, through the use of a coupling means, couple a defined radiation into and out of the window, whose wetting by moisture or dirt accumulation is to be measured. The transmitters are arranged on the coupling means, concentrically around the receiver or in concentric segments, where the coupling means are embodied in the form of a circle or ring. The transmitters, receivers, and coupling means are consequently mounted on a circular base area. As a result, the sensor or the sensor housing is embodied in the form of a circular cylinder.
One advantage of the sensor arrangement according to the invention, with the features of the main claim, is a significant improvement in the response characteristics of the sensor arrangement, which arises due to the increase in the ratio of the sensitive area to the supporting area. This is achieved by means of the trapezoidal arrangement the transmitters and receivers, since this produces an oblong, coherent sensitive area which increases the probability that a raindrop disposed on the window will drift onto the sensitive area, impelled by the relative wind. This results in a considerable improvement in the response characteristics.
Other advantages of the invention ensue from the characterizing features of the dependent claims.
Through the use of a number of receivers per transmitter, the number of measurement paths and therefore the number of sensitive areas is increased, which achieves a cost advantage. The same effect occurs when several transmitters and only one receiver are used. If two transmitters and two receivers are used, then four measurement paths and therefore four sensitive areas can be produced. If the distance of the two transmitters is approximately twice the distance of the two receivers, this results in a particularly uniform arrangement of the sensitive areas.
It also turns out to be advantageous to fasten the sensor arrangement to the window in such a way that in the installed position, the transmitters are disposed on the lower parallel of the trapezoid and the receivers are disposed on its upper parallel. Solar radiation onto the receivers, which preferably comes from above and represents an interfering external light, can thus be minimized.
As a result of the improved properties, in particular due to the improved ratio between the sensitive area and the supporting area, the supporting area of the sensor can be reduced, as a result of which the external dimensions of the sensor on the window are reduced. In this connection, it is particularly advantageous that due to the trapezoidal arrangement of the transmitters and receivers, the outer dimensions of the sensor housing can nevertheless be chosen as rectangular so that by making optimal use of the supporting area of the optical body, the base surface can be disposed within this supporting area. A rectangular supporting area and housing also have a cost-reducing effect on production. In addition, the smaller supporting area of the optical body represents a significant installation advantage since with a constant contact force, the contact pressure increases and consequently, an interfering formation of bubbles between the coupling medium and the window can be avoided.
It also turns out to be advantageous that the two transmitters and the two receivers are respectively disposed on the opposite, parallel sides in the arrangement. Since this produces four measurement paths, i.e. four sensitive areas of the sensor with only two transmitters and two receivers, this achieves a significant cost advantage.
An advantageous modification of the sensor arrangement has turned out to be one in which an automatic light control (ALC), for example of the kind described in DE 196 30 216 C2, is integrated into the rain sensor module. The receiving optics of the automatic light control (ALC) can detect a wide surrounding field by means of a directional and global sensing mechanism. Thus, for example, the forward-facing detection cones of the directional sensing mechanism can promptly detect tunnel entrances before the vehicle actually enters them. The trapezoidal arrangement of the transmitters and receivers, leaves sufficient space in the center of the sensor for the receiving optics, despite the compact design. In this connection, it turns out to be advantageous that the trapezoid has a larger parallel and a smaller parallel since the receiving optics can thus extend in the direction of the greater parallel without increasing the overall dimensions of the sensor.